[doc] Switch from pipes to backticks in comments
[doc] Switch from pipes to backticks in comments

The aim of this switch is to ensure we can render monospaced text (and
other markdown) in C and C++ api comments. Doxygen will render markdown
in code comments, but doesn't understand pipe characters.

Signed-off-by: Sam Elliott <32a4841301e2f444934d6e427a2764482fa7a43d@lowrisc.org>

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